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Lecture 9, Chapter 11
Terms in this set (22)
What's the difference between vector and rasters?
A vector has points and x,y,z coordinates that behave as discrete objects with spatial information and boundaries
A raster uses cells within grids to represent continuously changing objets such as satellite images
Each pixel contains...
one numeric value which represent some property of that pixel area (e.g. elevation or rainfall).
More bytes - larger numbers = more space
What's the difference between a cell/pixel and a polygon?
Unlike a polygon, each cell has only ONE attribute; its value.
What are the types of raster data?
How do they differ?
Give examples of each
Discrete (roads, land use) & continuous (DEM, image)
- Discrete data represents discrete objects such as LINES or POLYGONS, it only takes on relatively few values and adjacent cells often have same values. (lakes and stateS)
- Continuous data contains thousands or millions of potential values, and few adjacent cells have same values. (temperature and elevation)
What are bands?
Bands are multiple arrays that a single raster may include. Most often used to store color, images, and satellite images
Positive and negatives of using rasters
- easier at processing and analyzing, logarithms not so complicated
- good at displaying continuous data
- Trade off between quality and storage
- display of topology poorly
- store few attributes
The highest the resolution, the more storage is occupied.
Resolution is measured by cell size dimensions
In order for raster analysis to work..
Cells must be the same size and they must line up spatially
What happens if the input grids don't match?
If input grids don't match, then one must be resampled to match the other. Resampling can degrade the accuracy of a raster even if the difference in cell size and location is small.
What are the three resampling methods?
1) Nearest neighbor resampling
2) Bilinear resampling
3) Cubic convolution resampling
Explain the nearest neighbor resampling
- Grabs value from the old cell that falls at the center of the new cell.
- Preserves original value and should be used with categorical data
- FASTEST method
Explain bilinear resampling
Calculates a new value from the four cells that fall closest to the new cell.
Best with continuous data
Explain convolution resampling
Calculates a new value from the sixteen cells that fall closest to the center of the new cell
Best with continuous data
Model equation definition
Complex expressions with multiple inputs to calculate risk or hazard index
Logical operators definition
What type of raster represent logical operators?
Logical operators produce either TRUE or FALSE values in the output grid, based on whether the call makes the criterion
Boolean rasters represent logical operators
Drawbacks of boolean overlay
Criteria are simply true/false
Does not support gradation
Assumes all layer are equally important
Convert one set of grid values to another
- starts from a set of features (points, lines, polygons)
- creates a grid where each cell represents distance to the closest feature. distance are given in coordinate systen map units
*Indicates direction of travel to the closest road.
What's interpolation ?
Interpolation is the prediction of values in between measured points
What's the difference between density functions and interpolation?
interpolation predicts values in between measured points
Density functions count occurrences within a given radius and divide by the area
What's a zone?
A zone is the area of a raster or a data set that share the same INTEGER VALUE
What's a zonal function?
Examines and manipulates raster values in one within set of zones specified by another layer
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